The present invention relates to a radiation-type apparatus for measuring the basis weight of a sheet of material and, more particularly, to a means for accurately measuring the gap between the radiation source and the radiation sensor and compensating for variations therein.
Systems for measuring certain parameters of a moving sheet of material are well known in the art. For example, a system for measuring basis weight employs a gauge disposed with one part thereof on either side of the moving sheet, the gauge being mounted in a frame and adapted to travel transversely of the sheet. A radiation source is located in the lower gauge portion and a radiation detector is located in the upper gauge portion to receive radiation from the source. The amount of radiation received by the sensor is related to the basis weight of the sheet, and thus the gauging system monitors the basis weight as the sheet travels between the two gauge portions.
Parameters other than the basis weight of the sheet can be measured by systems similar to the one above-described. For example, the moisture content of a sheet of material may be measured by a device including a source of infrared radiation located in a gauging head member disposed on one side of the sheet and a detector or receiver of the radiation located in a head member disposed on the other side of the sheet. Opacity of the sheet can also be measured by a similar system.
In gauging systems of this type, the accuracy of the parameter measurement may be severely affected by variation in the distance or gap between the two gauging head parts. While means are known to correct the measured parameter values to compensate for such variations, such means are dependent upon accurate distance or gap measurement. One such method, described in U.S. Pat. No. 4,678,915, involves a magnetic measurement of the gap utilizing a transducer system mounted with the other sensors on the gauging head. Such a system, however, has significant limitations in that temperature effects and magnetic sensor accuracy are both proportional to the distance being measured. Thus, when measuring the distance across a relatively large gap, it is difficult to precisely measure small variations therein. The effects of lateral displacements on measurement accuracy also become more pronounced as gap distance increases.
It is, therefore, a primary object of the present invention to provide an improved basis weight measuring device, of the type comprising a radiation source and a radiation sensor spaced therefrom across a gap, including means for precisely measuring variations in the gap between the source and the sensor and for adjusting the sensor output signal in response thereto.
It is a further object to provide an improved device for precisely measuring the gap between the parts of a gauging head of a sheet-making machine and, more particularly, to provide such a device which minimizes or eliminates the effects of temperature variations and lateral displacements.
These objects are achieved, in general, by a basis weight measuring device of the aforementioned type wherein caliper means are provided to measure the thickness of the sheet, a portion of the caliper riding on the sheet and urging it against a first side of the gap, and second measurement means are provided to measure the distance between a reference point on the sheet-riding portion of the caliper means and the second side of the gap. The gap dimension is thus the sum of the thickness of the sheet, the distance between the base of the sheet-riding portion of the caliper means and the reference point thereon, and the distance between the caliper reference point and the second side of the gap. The first of these components may be precisely measured by conventional thickness gauges, such as the AccuRay Caliper sensor sold by ABB Process Automation Inc. The second component is a predetermined constant, defined by the sensor geometry and system dimensions. The third component may be measured by a conventional linear variable differential transformer (L.V.D.T.) such as the Model 100HR sold by Schaevitz Corp. Because this third component has relatively small variability, typically 1/8 inch or less, the aforementioned measurement limitations are avoided and great precision is achieved.